3. THE HIGHLIGHTS

So what are the most important scientific results from HDF so far? I
only have sufficient
time to sketch what I consider to be the five most significant results
whilst giving due
credit to earlier workers who established the foundations of what has
emerged. I have
to be a bit selective so I present this as a personal account rather
than a comprehensive review.

3.1. The flattening of the count slope N(m)

The quest to take deeper images of the sky motivated the HDF at its most
basic level but
this quest has a long and distinguished history.
Sandage (1995) discusses the classical work, whereas
Koo & Kron (1992)
and Ellis (1997)
review the more recent observations.
The modern era begins with the commissioning of the wide-field prime focii on
our national 4-m telescopes in the 1970's. Combining fine-grain
emulsions and automatic
measuring machines, Kron, Kibblewhite and Tyson laid the foundations of image
processing of faint galaxy images
(Kron 1978,
Peterson et al. 1979,
Tyson & Jarvis 1979).
In an era when the photographic plate is so often disregarded, it is
salutary to note how
much of our observational achievement was established from photographic
plates. Only
now, after 20 years, are giant CCD cameras rivaling the combined depth
and field of view.
Of particular note for this meeting is Koo's thesis (1981) where, in an
early version of the
HDF, four-colour photographic photometry was analyzed in the context of
photometric
redshifts to demonstrate enhanced star formation as a function of
look-back time.

The importance of pushing deeper was obviously recognized.
Tyson (1988)
was the first
to attempt ambitious long CCD exposures developing, with Jarvis, Valdes,
Seitzer and
others, the relevant observing and processing technologies. The steep
blue count slope first found by
Kron (1978)
seemed to continue and many of us imagined we might soon
hit the confusion limit. The suggestion that the count slope flattened
below the Olber's limit, dlogN / dm = 0.4, came tentatively from
Lilly et al. (1991)
and later, with greater confidence, from the very deep exposures taken by
Metcalfe et al. (1995)
including the
Herschel Deep Field, a friendly ground-based rival of the
HDF. (1)

Figure 1. Differential number magnitude
counts in the B-band (including those derived from
the HDF) and K-band (including recent Keck determinations using the Keck
telescope). The
K counts have been offset by +1 dex for clarity. The two power law
slopes (dashed lines) indicate
the point beyond which the integrated night sky brightness begins to
converge. Solid lines refer
to no evolution predictions in the Einstein-de Sitter case (see
Ellis 1997
for further details).

The flattening was dramatically confirmed in the HDF counts and overcounting
multi-component galaxies as separate units
(Colley et al. 1996)
would make the faint slopes
even flatter. The bulk of the received extragalactic light must
therefore come from the
point of inflexion - an apparent magnitude (B
25) within
spectroscopic reach where
the mean redshift is modest (z
1). Importantly, the same effect has been seen in the
infrared at K 18
(Gardner et al. 1993,
Moustakas et al. 1997).
Very few of the faint
K-selected galaxies are not visible in the optical suggesting the result
is a fundamental feature of galactic history.

1I should add, somewhat topically, that
Tom Shanks looked like doing marvelously well out
of the recent British election as his Herschel Deep Field blue galaxies
were adopted as a slogan by
the Conservative party whose emblem was projected on them in a national
newspaper. However,
the colour of the British sky has since switched dramatically from blue
to red! Back.